Prevention and treatment of cancer has significantly improved in the United States during the past decade because of advancements in epidemiology, the technology of treatment, and the ability to deliver earlier diagnosis. Finding a cure for a diversity of cancers, such as lung, breast, prostate, colon and others however, is still a major challenge. Current approaches for the treatment of cancers are, however, still limited to the lengthening of life, or the increase in the quality of life. Additionally, most meaningful therapeutics still have significant side effects. Therefore, it is imperative to find more effective therapeutic agents with lower side effects.
Tumor cells are characterized by uncontrolled cell proliferation due to the loss of the integration and coordination of extracellular signals with the cell cycle machinery. A typical cell cycle is classified into G1, S, G2 and M phases [1-3] and is illustrated in FIG. 2. In mammalian cells, proliferation is controlled in the G1 phase of the cell cycle. At the restriction point, cells can have different destinies. Examples of these cell destinies include: 1) leaving the cell cycle and entering a reversible quiescence phase; 2) exiting cell cycle and undergoing apoptosis; 3) differentiating and irreversibly exiting from the cell cycle; and 4) passing through the restriction point and becoming largely independent of extracellular signals and progress automatically through subsequent cell cycle phases (S, G2, M) to the next G1 phase. A variety of proteins are in turn responsible for the regulated progression of cells through the cell cycle. The key components of cell cycle machinery are the cyclins, the cyclin-dependent kinases (CDKs) and their inhibitors. Cyclins are a remarkably diverse family of proteins, which are synthesized from the mid/late of G1 phase till the M phase of the cell cycle and then rapidly degraded. A CDK typically contains a catalytic domain of 300 amino acids, which is inactive by it self. Cdks become active by binding to a cyclin. Activity of cdks is inhibited by their endogenous inhibitors (cdk inhibitors, or cdkIs include p15/p16/p18/p19 and p21/p27). Specific cyclin/CDK complexes are formed at specific stages of the cell cycle and their activities are required for progression of the cell cycle through S phase and mitosis.
Over-activation of CDKs is a character of a majority of human tumor cells. Strategies have been developed to modulate CDK activity for therapeutic intervention by either directly targeting the catalytic CDK subunit or indirectly affecting the CDK regulatory pathways [3]. Small molecule CDK inhibitors were designed to interact specifically with the ATP binding site of CDKs, such as flavopiridol congeners, polysulfates, toyocamycin derivatives, etc. Anticancer effects have been shown in clinical trials for those agents. Modulation of CDK activities can be achieved by regulating phosphorylation of CDKs or altering the expression of the CDKs or the their inhibitors (CKDIs). It is difficult to find specific modulators that do not interfere with other cell cycle components and do not affect normal cells.
A need thus exists for compounds that are easily produced and are highly effective at treating cancer but have minimal toxicity to normal cells.
Many Chinese herbs contain potent anti-cancer chemical components. For example, several Chinese plants such as Camptotheca acuminata (camptothecin), Cephalotaxus sp. (homoharringtonine/harringtonine) have provided compounds with significant antitumor activity [6]. PC-SPES, a mixture consisting of extracts from eight herbs for the treatment of prostate cancer, has been demonstrated to have potent anti-androgen activity [7]. Huanglian, a Chinese herbal extract, has recently been shown to inhibit cell growth by suppressing the expression of cyclin B1 and inhibiting CDC2 kinase activity in human cancer cells [8]. In addition, many Chinese herbal products have demonstrated an important role in cancer chemoprevention.
Studies have recently demonstrated that indirubin molecules from the anticancer Chinese herb—Qing Dai, exhibit their anticancer activity through modulating cyclin-dependent kinases [9].
Our previous studies demonstrated that meisoindigo, a second generation of indirubins, arrests leukemia cells at G1 phase, inhibits expression of oncogene c-myb, and induces cell differentiation and maturation at low concentrations (low toxicity) in which cell growth is completely inhibited without a decrease in cell viability [10, 11]. Recent studies demonstrate that indirubin selectively inhibits cyclin-dependent kinases (cdks) by competing with ATP for binding to the catalytic site of the kinase (FIG. 2) [12].